1. Field of the Invention
The present invention relates to an image processing method, a printer (printing apparatus) and a storage medium, and more particularly to techniques of reducing the so-called granularity of a color image printed by using a plurality of different color materials.
The invention is applicable to all apparatuses using printing media such as paper, leather, unwoven cloth, OHP sheets, and metal. Specific examples of such apparatuses include business machines such as printers, copy machines and facsimiles, industrial machines such as printing machines and textile printing machines, and the like.
2. Related Background Art
Information processing apparatuses such as copy machines, word processors and computers, and communication apparatuses are widely used, and apparatuses for printing data from such information processing apparatuses, for example, ink jet type printers are prevalent. In order to improve the print speed of such an ink jet type printer, a print head having a plurality of ink jet nozzles integrally disposed is generally used. Such a print head capable of printing different colors has recently come to be widely used.
In ink jet printing, ink is used as the recording liquid and is jetted out from an ink discharge opening of a print head to be deposited on a printing medium such as a paper sheet to form a print dot. Since this is a non-contact printing method, this method has the advantage that noise is low. It is also easy to dispose nozzles of a print head at a high density so that a printed image can have a high resolution and the print speed can be made high. A color image can be printed with a relatively simple head structure, and the apparatus itself can be made compact and simple. In addition, a high-quality and inexpensive image printed on a printing medium such as a plain paper sheet can be obtained without any particular additional processes such as development and fixing. Since this method has many advantages, as described above, it is predominant nowadays. High image quality and high print speed are desired more and more with the advent of color printing.
As the ink jet printing method of printing a color image with three colors of inks, cyan (C), magenta (M) and yellow (Y), or with black (K) ink as well (three- or and four-color printing, respectively), many methods have been proposed to form a multi-tonal image.
With one method which has been adopted by many printers, the size of the dots formed on a printing sheet with discharged ink is maintained constant, and the tonal level of an image to be printed is changed by changing the dot density (dot discharge frequency per unit area). With another method, the diameter of a dot to be formed on a printing sheet is adjusted from dot to dot, so that the density per unit area varies within the image.
With recent developments of fine working of a head for generating an ink droplet, the number of dots (dot density: dpi) per unit length and the variable range of a dot diameter are being improved year after year. In the case of an ink jet printer, the print density (resolution) is about 300 dpi to 1200 dpi, and the ink droplet diameter is several tens of microns. This printing performance is far inferior to a silver-salt photograph (it is said that the resolution of an image on film is several thousand dpi).
In a low density area of a print image formed by an ink jet printer, i.e., in an area having a low dot density, the ink dots are relatively widely dispersed, and the print image may show conspicuous granularity (a grainy appearance). The positions and distribution of dots are changed as a result of binarization with certain binarizing methods, such as the dither method, even within an area where the print density is uniform. Therefore, even in a low dot density area where dots are dispersed, dots may be formed relatively tightly clustered in a small area and not distributed uniformly. In such a case, granularity becomes conspicuous.
With a conventional binarizing method, a printer halftone process has been devised in such a manner that the distribution of dots of each color in a low print density area is controlled to prevent clustering of dots of that color, as much as possible, to reduce the granularity.
However, granularity may become conspicuous even if such conventional method is used. This is because, even if the distribution of color dots is controlled to prevent clustering, this method is performed separately for each color, and it happens that the distributions of cyan and magenta are localized in some cases so that a viewer sees granularity. Although there is an approach that involves preventing clustering of the dots of cyan and magenta taken together, the resulting reduction of granularity is not sufficient if the dot diameter is relatively large.
With another known method, the granularity is reduced by using ink whose color material has a low density, such as a dye. Using ink having a low density in addition to ink of respective colors, however, may result in an increased cost of the printing apparatus.